When a calling party (whether a subscriber within the mobile telecommunications network or outside it) attempts to call a mobile terminal within the network, that mobile terminal must be paged. Paging is a process of broadcasting a message which alerts a specific mobile terminal to take some action, such as to notify the terminal that there is an incoming call to be received.
The architecture and steps taken when an incoming call is directed to a mobile communication device across a GSM/UMTS macro network are shown in FIGS. 1 and 2.                1. At step 100, a call is initiated for a target mobile subscriber B. The call may have been initiated by another mobile or from a fixed line device. Typically, this call is initiated by entering the unique contact number (for example MSISDN) of subscriber B.        2. The carrier from where the call is initiated, or an interconnecting carrier, identifies a particular mobile operator that the contact number for subscriber B is assigned to (step 110). The carrier identifies a Gateway Mobile Switching Centre (GMSC) 210 associated with that mobile operator within the core network 12 and contacts that GMSC 210 (step 120). Gateway MSC 210 then takes on responsibility for routing the call to the target subscriber.        3. If the GMSC 210 does not know the subscriber, which is typically the case, it queries the Home Location Register (HLR) 10 associated with the mobile operator to which B is a subscriber to determine the Mobile Switching Centre (MSC), in this case MSC 2, which is currently handling incoming and outgoing communication for subscriber B and to which subscriber B is currently registered (step 130). HLR 10 stores the details of the MSCs which are currently handling communications for each of its subscribers.        4. At 140 GMSC 210 forwards the call to MSC 2 which is currently handling communications to and from target subscriber B. The control of the call is passed to the MSC.        5. At 150 the MSC identifies the target subscriber and determines from its Visitor Location Register (VLR) 14 in which Location Area (LA) subscriber B is located. A location area is a geographic area covered by at least one cell. A MSC covers at least one location area.        6. At 160, MSC 2 instructs the BSC(s) and/or RNC(s) parenting cells belonging to that LA to page the subscriber on all cells within the location area in which the subscriber is located.        7. Paging messages are then transmitted on the paging channels of all the cells of the LA in which the customer is located.        8. The target subscriber receives and reads the paging message and responds at 170, setting up a radio signalling channel and progressing with the call control.        9. The call is established at 180 and a radio voice channel is established.        
Typically there are tens of MSCs in each country, each being responsible for several cells in at least one LA. Thus when an incoming call is directed to a mobile subscriber, only a fraction of the total number of cells in the network are paged (i.e. those in the relevant LA under control of the relevant MSC). In this way, pages for a particular subscriber are not transmitted in cells under MSCs which do not contain the subscriber.
There have recently been proposals to allow access to the features and services provided by GSM and UMTS networks other than by accessing those networks in the conventional manner described above. It has been proposed to increase network capacity by providing additional special base stations (micro base stations), variously referred to as access points (APs), home access points (HAPs) or femto-cells, amongst others. These APs are typically located at a subscriber's home. It has also been proposed to use APs in the Long Term Evolution (LTE) telecommunications network currently being developed, but not yet implemented. LTE is likely to be the next network implementation after 3G.
APs connect to the core network of a mobile communication network via IP networks. Typically, APs will be available to mobile subscribers to plug into their home DSL connection and network signals are carried to and from the AP via the IP network. The AP will, typically, provide mobile network coverage throughout the home, but will have a small coverage area. Thus subscribers will camp on their AP only when in their home.
An advantage of using an access point connected to the core network via an IP network is that existing broadband DSL connections can be used to link mobile terminals with the network core without using the capacity of the radio access network or transmission network of a mobile telecommunications network, or they are able to provide mobile network access where there is no conventional radio access network coverage. For example, UMTS coverage could be provided by an access point where there is no conventional UMTS coverage (perhaps only GSM coverage).
Since these access points are not conventional base stations, however, additional challenges arise. In particular, communications between the access point and the network are IP based communications, and may be, for example, transmitted via an ADSL backhaul connection to an IP network, such as the Internet. However, the capacity of such ADSL backhauls are limited, and not under the control of the telecommunications network provider, so there needs to be restrictions put in place on which user terminals (UEs) are able to access each AP in order to maintain an acceptable degree of service quality.
A problem that arises because of the need of such access restrictions is that UEs will be split into those which are authorised to access each AP and those which are not authorised to. Nevertheless all UEs would attempt to access an AP once it became available as a preferred base station.
For instance, considering a UE in a connected state, when an AP provides the UE with the best signal strength in a given region, the UE's current Node B/base station will attempt to hand the UE over to the AP. If the UE is one of the UEs which is not supposed to use the AP, the handover procedure will be initiated and the core network or the AP will have to reject the handover. Similarly, considering a UE in an idle state, when the UE receives the Location Area broadcast by the AP, and notes it to be different from its previous Location Area, the UE will look to camp on the AP and request a Location Area update from the network. If the UE is not authorized to camp on the AP the update will be rejected by the core network.
These access rejections are a waste of signalling resources. This is particularly a problem where an AP exists in an area of high traffic or high penetration of APs, as there is likely to be an unacceptable level of access attempts by UEs, many of which will result in unnecessary signalling. There therefore needs to be a better balance between restricting access to APs and limiting the use of signalling resources in doing so.
A further problem faced by mobile communication networks is how to handle paging through APs.
By definition, the location of mobiles on idle mode is only known with the resolution of the LA. When a mobile moves between LAs, i.e. when it reselects a cell belonging to a different LA to that where it is registered, it initiates a LA update procedure to inform the network about such a move.
Therefore, when a call has to be terminated towards a mobile in idle mode, the phone needs to be paged on all cells of the LA. The load of the paging channels in any given cell is proportional to the number of cells in the location area, so when numerous APs are added to the network, the network load naturally increases. However, if this load becomes too high, the LA would need to be split or cells re-parented into another LA.
Due to the limitations of the network and to the expectation that each RNC concentrator will control in the order of hundreds of thousands of APs, it is likely that a high number of APs may belong to the same LA. Mobile terminal traffic will therefore create a considerable paging load on the femto-layer.
These problems are addressed by the present invention.